W Hudson Bay freeze-up earlier than average for 2nd year in a row, polar bear hunt resumes

This is the second year in a row that freeze-up of Western Hudson Bay ice has come earlier than average. Movement of tagged bears and reports by folks on the ground in WH show some polar bears are starting to hunt seals on the sea ice that’s developing along the shore. It’s unlikely that a strong wind will again blow the newly-formed ice offshore (as happened earlier this year) because the ice is more extensive. It seems polar bear viewing season in Churchill will be ending early this year, just like it did last year.

Tundra Buggy Cam_10 Nov 2017_bear headed offshore pm

The 9 November map Andrew Derocher (University of Albera) published on twitter showing tagged and collared polar bear movements on Hudson Bay makes it look like almost no ice is present:

However, the Canadian Ice Service chart for 10 November shows the ice very clearly:

Sea ice Canada 2018 Nov 10

Ice has been developing rapidly over the last couple of days, so ice graphs published for the week of 5 November that show ice coverage below 5% are no longer relevant (see below, where 2018 has the same amount of coverage as 2017 for that week):

Hudson Bay sea ice same week with average at Nov 5 2018

The ice concentration chart for Friday (9 Nov) is shown below (I’ll update this over the next few days). See the ice when bears left last year here.

Hudson Bay NW daily stage of development 2018 Nov 10

It’s cold in Churchill today, so the ice should thicken and expand over the weekend:

Churchill weather 2018 Nov 10 at 10 am PT

Like Derocher’s student Laura Castro de la Guardia, I am using a definition of “freeze-up” that describes the behaviour of polar bears to newly formed ice, not the date when fall ice coverage on the bay reaches 50% (e.g. Lunn et al. 2016).

According to a recalculation of data that goes up to 2015 and back to 1979 (Castro de la Guardia 2017), in the 1980s bears left the ice at freeze-up (10% sea ice coverage in WHB) about 16 November ± 5 days while in recent years (2004-2008) they left about 24 November ± 8 days, a difference of 8 days.

The relative change in the dates that bears left the shore between the 1980s and recent years is only about 1 week, on average (with lots of variation).

Freeze-up dates of 10-12 November or so (Day 314-316) for 2017 and 2018 are therefore two of the earliest freeze-up dates since 1979 (the earliest being 6 November, Day 310, in 1991 and 1993), see Fig 3 from Castro de la Guardia 2017 below.

castro-de-la-guardia-et-al-derocher-2017-fig-3-no-caption
Figure 3 from Castro de la Guardia (2017) showing freeze-up and breakup dates and ice-free days 1979-2015 for Western Hudson Bay, showing that the earliest freeze-up dates since 1979 (top panel) came on 6 November, Day 310 (in 1991 and 1993).

Virtually all Western Hudson Bay bears leave the shore within about 2 days of sea ice concentration reaching 10% (Castro de la Guardia 2017; Cherry et al. 2013), although as noted above, Southern Hudson Bay bears leave when it reaches about 5%: in other words, the bears go as soon as they possibly can.

As I discussed last year regarding newly-published studies (Obbard et al. 2015, 2016) on the status of Southern Hudson Bay (SH) bears:

“…SH polar bears left the ice (or returned to it) when the average ice cover near the coast was about 5%. This finding is yet more evidence that the meteorological definition of “breakup” (date of 50% ice cover) used by many researchers (see discussion here) is not appropriate for describing the seasonal movements of polar bears on and off shore.”

That 2017 post (with its list of references) is worth another look for its discussion of the following points: the definition of freeze-up; the relationship of official freeze-up and breakup dates to the dates that bears depart; the overall health and survival of Western and Southern Hudson Bay polar bears.

A final note: if PBI spokesperson Amstrup had been right about his predictions of Arctic sea ice and polar bear survival back in 2007 when he was the head of the US Geological Survey’s polar bear research team, there would be no polar bears at all in Hudson Bay right now (Crockford 2017), not a thriving population of fat, healthy bears.

References

Castro de la Guardia, L., Myers, P.G., Derocher, A.E., Lunn, N.J., Terwisscha van Scheltinga, A.D. 2017. Sea ice cycle in western Hudson Bay, Canada, from a polar bear perspective. Marine Ecology Progress Series 564: 225–233. http://www.int-res.com/abstracts/meps/v564/p225-233/

Cherry, S.G., Derocher, A.E., Thiemann, G.W., Lunn, N.J. 2013. Migration phenology and seasonal fidelity of an Arctic marine predator in relation to sea ice dynamics. Journal of Animal Ecology 82:912-921. http://onlinelibrary.wiley.com/doi/10.1111/1365-2656.12050/abstract

Crockford, S.J. 2017. Testing the hypothesis that routine sea ice coverage of 3-5 mkm2 results in a greater than 30% decline in population size of polar bears (Ursus maritimus). PeerJ Preprints 19 January 2017. Doi: 10.7287/peerj.preprints.2737v1 Open access. https://peerj.com/preprints/2737/

Lunn, N.J., Servanty, S., Regehr, E.V., Converse, S.J., Richardson, E. and Stirling, I. 2016. Demography of an apex predator at the edge of its range – impacts of changing sea ice on polar bears in Hudson Bay. Ecological Applications, in press. DOI: 10.1890/15-1256

Obbard, M.E., Stapleton, S., Middel, K.R., Thibault, I., Brodeur, V. and Jutras, C. 2015. Estimating the abundance of the Southern Hudson Bay polar bear subpopulation with aerial surveys. Polar Biology 38:1713-1725.

Obbard, M.E., Cattet, M.R.I., Howe, E.J., Middel, K.R., Newton, E.J., Kolenosky, G.B., Abraham, K.F. and Greenwood, C.J. 2016. Trends in body condition in polar bears (Ursus maritimus) from the Southern Hudson Bay subpopulation in relation to changes in sea ice. Arctic Science, in press. 10.1139/AS-2015-0027

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November 10, 2018 at 07:19PM

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